It has been known that 4-methylpyridazin-3-one compounds of the formula (I): ##STR3## [wherein, X represents a hydrogen or fluorine atom, and Y represents a hydrogen atom, C.sub.1 -C.sub.6 alkoxy, C.sub.3 -C.sub.6 alkenyloxy or C.sub.3-C.sub.6 alkynyloxy group] have excellent herbicidal activity (International patent publication No. WO97/07104), and it has been demanded to develop an effective method to produce these 4-methylpyridazin-3-one compounds.